Amplifier circuit having a compensating amplifier unit for improving loop gain and linearity

ABSTRACT

An amplifier circuit includes a feedback loop; a former stage amplifier unit coupled to an input node of the amplifier circuit and the feedback loop for amplifying the difference between an input signal received from the input node and an output signal which comes back through the feedback loop, and outputting a first output signal; a compensating amplifier unit coupled to the former stage amplifier unit for amplifying the first output signal and outputting a second output signal; and an output stage amplifier unit coupled to the compensating amplifier unit and the feedback loop for outputting a third output signal according to the second output signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an amplifier circuit, and more particularly, toan amplifier circuit having a compensating amplifier unit for improvingloop gain and reducing distortion.

2. Description of the Prior Art

The negative feedback mechanism that is capable of improving systemlinearity is broadly applied in the electronics industry. Please referto FIG. 1. FIG. 1 is a schematic diagram illustrating a conventionalfeedback system 100. The feedback system 100 can be achieved utilizingan open-loop gain A(s) and a feedback factor β. As one of ordinary skillin the pertinent art will understand, the relationship between an outputvoltage V_(o) and an input voltage V_(i) of the feedback system 100 isshown as follows: $\begin{matrix}{\frac{Vo}{Vi} = \frac{A(s)}{1 + {{A(s)}\beta}}} & {{Formula}\quad 1}\end{matrix}$

Oftentimes, when the value of the loop gain A(s)β is much larger than 1,the relationship between the output voltage V_(o) and the input voltageV_(i) of the feedback system 100 can be simplified as follows in Formula2: $\begin{matrix}{{\frac{Vo}{Vi} = {\frac{1}{\beta}\quad{as}\quad{A(s)}\beta}}\operatorname{>>}1} & {{Formula}\quad 2}\end{matrix}$

The feedback factor, β, is a frequency irrelevant term. It means that Vois proportional to Vi in all frequencies and the gain is a constant,$\frac{1}{\beta}$if only if the loop gain A(s)β is much larger than 1. However, if thefeedback factor β is increased, the close-loop gain $\frac{1}{\beta}$is inevitably decreasing. Given this relationship, the feedback factor βcannot be increased in an unlimited or unbounded fashion as a method forincreasing the loop gain A(s)β of the feedback system 100. Therefore,one of the practical means for increasing the loop gain A(S)β of thefeedback system 100 is by increasing the open-loop gain A(s).

Please refer to FIG. 2. FIG. 2 is a circuit diagram of a typicalamplifier circuit 200. The amplifier circuit 200 includes an integratorcircuit 210, an output stage amplifier unit 220, and a feedback resistorR_(f). The integrator circuit 210 includes an operational amplifier 212,a resistor R₁, and a capacitor C₁. In the amplifier circuit 200, theoutput stage amplifier unit 220 is commonly realized as an output stagewith stable and fixed gain G, and the value of the gain G is designedaccording to various operational requirements and noise-rejectionlimitation. Generally, the value of the gain G is not large enough toserve as an open loop gain stage along. In order to increase theopen-loop gain A(s) in an effort to improve the system linearity, themethod of related art will increase the open-loop gain by A(s) couplingthe integrator circuit 210 to the output stage amplifier unit 220.Adding the integrator circuit 210 not only increases the open-loop gainA(s) but also serves the additional purpose of filtering unnecessaryhigh-frequency noise. As one of ordinary skill in the pertinent art willunderstand, the relationship between an output voltage V_(o) and aninput voltage V_(i) of the amplifier circuit 200 is shown as follows:$\begin{matrix}{\frac{Vo}{Vi} = {\frac{- \frac{G}{{sC}_{1}R_{1}}}{1 + \frac{G}{{sC}_{1}R_{f}}} = {\frac{- \frac{G}{{sC}_{1}R_{1}}}{1 + {( {- \frac{G}{{sC}_{1}R_{1}}} )( {- \frac{R_{1}}{R_{f}}} )}} = \frac{A(s)}{1 + {{A(s)}\beta}}}}} & {{Formula}\quad 3}\end{matrix}$

wherein $\begin{matrix}{{{A(s)} = {- \frac{G}{{sC}_{1}R_{1}}}},{\beta = {- \frac{R_{1}}{R_{f}}}}} & {{Formula}\quad 4}\end{matrix}$Additionally, the loop gain A(s)β of the amplifier circuit 200 can bedetailed as shown below in Formula 5: $\begin{matrix}{{{A(s)}\beta} = \frac{G}{{sC}_{1}R_{f}}} & {{Formula}\quad 5}\end{matrix}$

As shown in Formula 5, in the context of the related art, the loop gainA(s)β of the amplifier circuit 200 can be increased to improve thesystem linearity by increasing the gain G of the output stage amplifierunit 220 or by either decreasing the capacitor C₁ of the integratorcircuit 210 or decreasing the resistance value of feedback resistorR_(f). As mentioned previously, the gain G is usually small. And becauseof system stability issues and limitations of the operational frequencyrange, the capacitor C₁ of the integrator circuit 210 or the resistanceof feedback resistor R_(f) cannot be decreased unlimitedly or in anunbounded fashion. In consideration of these factors, the loop gainA(s)β of the prior art is limited to a certain level and the linearityof the system can't not be further improved.

SUMMARY OF THE INVENTION

It is therefore one of the many objectives of the claimed invention toprovide a compensating amplifier unit in an amplifier circuit forincreasing the loop gain and improving the system linearity.

According to an aspect of the claimed invention, an amplifier circuit isdisclosed. The amplifier circuit includes a feedback loop; a formerstage amplifier unit coupled to an input node of the amplifier circuitand the feedback loop for amplifying the difference between an inputsignal received from the input node and an output signal which comesback through the feedback loop, and outputting a first output signal; acompensating amplifier unit coupled to the former stage amplifier unitfor amplifying the first output signal and outputting a second outputsignal; and a output stage amplifier unit coupled to the compensatingamplifier unit and the feedback loop for outputting a third outputsignal according to the second output signal.

The amplifier circuit in the claimed invention provides the advantagesof reducing the system distortion by increasing the loop gain, andmeanwhile improving the system's ability to reject power noise.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a conventional feedbacksystem.

FIG. 2 is a circuit diagram of a typical amplifier circuit.

FIG. 3 is a block diagram of an amplifier circuit according to anembodiment of the present invention.

FIG. 4 is the loop-gain diagram comparing between the prior art and thepresent invention.

FIG. 5 is the PSRR-frequency related characteristic diagram comparingbetween the prior art and the present invention.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, consumer electronic equipment manufacturers may refer to acomponent by different names. This document does not intend todistinguish between components that differ in name but not function. Inthe following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .” The terms“couple” and “couples” are intended to mean either an indirect or adirect electrical connection. Thus, if a first device couples to asecond device, that connection may be through a direct electricalconnection, or through an indirect electrical connection via otherdevices and connections.

Please refer to FIG. 3. FIG. 3 is a block diagram of an amplifiercircuit 300 according to an embodiment of the present invention. Theamplifier circuit 300 includes a former stage amplifier unit 310, acompensating amplifier unit 330, an output stage amplifier unit 320, anda feedback resistor R_(f) (i.e., the feedback resistor R_(f) isperceived as a feedback loop). The former stage amplifier unit 310includes an operational amplifier 312, a resistor R₁, and a capacitorC₁. The compensating amplifier unit 330 includes an operationalamplifier 332, a plurality of resistors R₂, R₃, and a capacitor C₂. Theoutput stage amplifier unit 320 includes a comparator 322 and a drivercircuit 324. The comparator 332 is utilized to compare an output signalfrom the compensating amplifier unit 330 to a reference signal V_(T) forgenerating a pulse width modulated (PWM) signal. Next, the drivercircuit 324 is utilized to generate an output voltage V_(O) according tothe PWM signal. In this embodiment of the present invention, the formerstage amplifier unit 310 amplifies a difference (i.e. error) between aninput voltage V_(I) and an output voltage V_(O) which comes back throughthe feedback loop and outputs the amplified error voltage to thecompensating amplifier unit 330. The compensating amplifier unit 330further amplifies the error voltage and outputs the further amplifiedvoltage to the output stage amplifier unit 320. Please note that, inthis embodiment, both the former stage amplifier unit 310 and thecompensating amplifier unit 330 are composed of the integrator circuit.However, many other embodiments of the present disclosure that utilizedifferent amplifier application circuits are also possible. For example,to add a specific gain to the system, any circuit that is composed of anoperational amplifier can be utilized in the former stage amplifier unit310 or the compensating amplifier unit 330. By way of example, FIG. 3illustrates the amplifier circuit of the present invention and is notintended to represent or meant to be taken as implying any limitations.Additionally, except for the basic integrator circuit, the compensatingamplifier unit 330 further includes a resistor R₃. The related operationof the compensating amplifier unit 330 and included resistor R₃ isdetailed later. Finally, in this embodiment, the output stage amplifierunit 320 is achieved by a switching amplifier circuit. However, in otherembodiments of the present invention, the output stage amplifier unit320 can be realized in a broad sense by utilizing any electricapplication circuit with a fixed gain G. That is, other embodiments ofthe present disclosure, which include different fixed gain G based onvarious operational requirements and noise-rejection limitation, arealso possible.

In contrast to the related art, this embodiment in the present inventionincreases the loop gain A(s)β of the amplifier circuit 300 to improvethe system linearity by means of coupling the compensating amplifierunit 330 between the former stage amplifier unit 310 and the outputstage amplifier unit 320. Please refer to the above-mentioned Formula 3and Formula 4, the relationship between an output voltage V_(O) and aninput voltage V_(I) of the amplifier circuit 300 is shown as follows:$\begin{matrix}{\frac{Vo}{Vi} = {{- \frac{\frac{G( {1 + {{sC}_{2}R_{3}}} )}{s^{2}C_{2}R_{2}C_{1}R_{1}}}{1 + \frac{G( {1 + {{sC}_{2}R_{3}}} )}{s^{2}C_{2}R_{2}C_{1}R_{f}}}} = \frac{A(s)}{1 + {{A(s)}\beta}}}} & {{Formula}\quad 6}\end{matrix}$Additionally, the loop gain A(s)β of the amplifier circuit 300 can beexpressed as shown below: $\begin{matrix}{{{A(s)}\beta} = \frac{G( {1 + {{sC}_{2}R_{3}}} )}{s^{2}C_{2}R_{2}C_{1}R_{f}}} & {{Formula}\quad 7}\end{matrix}$

As shown in Formula 6 and Formula 7, in contrast to the related art,this embodiment in the present invention integrates the compensatingamplifier unit 330 between and coupled to the former stage amplifierunit 310 and the output stage amplifier unit 320. The product of theresistors R₂ and the capacitor C₂ of the compensating amplifier unit 330(i.e., the term C₂R₂ in Formula 7) is a value that is far less than 1.It can significantly increase the loop gain A(s)β of the amplifiercircuit 300 and therefore improve the system linearity. Additionally,except for the basic components of an integrator circuit (e.g., theoperational amplifier 332, the resistors R₂, and the capacitor C₂), thecompensating amplifier unit 330 further comprises a resistor R₃. Asshown in Formula 7, the resistor R₃ provides the amplifier circuit 300with a zero point in the frequency domain to increase the phase marginof the system. Please refer to FIG. 4. FIG. 4 is the Bode plot of theamplifier circuit 300 shown in FIG. 3. In FIG. 4, in addition to thedotted line B, which represents the loop gain-frequency relatedcharacteristic of the conventional amplifier circuit 200 shown in FIG.2, FIG. 4 includes the solid line A that represents the loopgain-frequency related characteristic of the amplifier circuit 300 inthe present invention as shown in FIG. 3. As shown in FIG. 4, in therequired working frequency range, the loop gain A(s)β of the amplifiercircuit 300 in the present invention is far greater than the loop gainof the conventional amplifier circuit 200. Besides, as mentioned above,the zero point in the frequency domain, which is provided from theresistor R₃, leads to the rate of convergence (ROC) after the zero pointis about −20 dB/Dec, and this leads to the phase of the system beinglarger than 180 degrees. That is, the zero point ensures the phasemargin is large enough (e.g., usually above 60 degrees), and thenguarantees the stability of the amplifier circuit 300.

Moreover, the amplifier circuit 300 in the present invention also canimprove the system's ability to reject the power noise, and thusincrease the power supply rejection ratio (PSRR) of the system. In thisembodiment, if the power noise of the system is V_(s) and therelationship between the output voltage V_(O) of the amplifier circuit300 and the power noise V_(s) can be expressed as follows:$\begin{matrix}{\frac{Vo}{Vs} = \frac{s^{2}C_{2}R_{2}C_{1}R_{f}}{{s^{2}C_{2}R_{2}C_{1}R_{f}} + {{sGC}_{2}R_{3}} + G}} & {{Formula}\quad 8}\end{matrix}$

Additionally, for the conventional amplifier circuit 200 shown in FIG.2, the relationship between the output voltage V_(O) of the amplifiercircuit 200 and the power noise V_(s) can be expressed as follows:$\begin{matrix}{\frac{Vo}{Vs} = \frac{s}{s + \frac{G}{C_{1}R_{f}}}} & {{Formula}\quad 9}\end{matrix}$

Please refer to FIG. 5. The solid line A shown in FIG. 5 represents thePSRR-frequency related characteristic of the amplifier circuit 300 inthe present invention and the dotted line B shown in FIG. 5 representsthe PSRR-frequency related characteristic of the conventional amplifiercircuit 200. The feature curves shown in FIG. 5 can be elicited from theFormula 8 and Formula 9. Please note that, since the method ofPSRR-frequency calculation is considered well known in the pertinentart, further details are therefore omitted for the sake of brevity. Asshown in FIG. 5, in the needed (i.e., required) working frequency range,the PSRR of the amplifier circuit 300 is far greater than the PSRR ofthe conventional amplifier circuit 200. That is, comparing with theoutput voltage V_(O) of the conventional amplifier circuit 200, theoutput voltage V_(O) of the amplifier circuit 300 is less affected bythe power noise V_(s) and therefore significantly improves thenoise-rejection ability.

As mentioned above, after the output stage amplifier unit 320 receivesthe further amplified error voltage outputted from the compensatingamplifier unit 330, the comparator 332 is utilized to compare thisvoltage with a reference signal V_(T) to generate a pulse widthmodulated (PWM) signal. In practice, the reference signal V_(T) could bea triangular signal or sawtooth signal. Next, the driver circuit 324 isutilized to generate an output voltage V_(O) according to the PWMsignal. According to the amplifier circuit 300 in the present invention,the switch amplifier circuit applied in the amplifier circuit 300reduces the system distortion by increasing the loop gain, and meanwhileimproving the system's ability to reject power noise.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. An amplifier circuit, comprising: a feedback loop; a former stageamplifier unit coupled to an input node of the amplifier circuit and thefeedback loop for amplifying the difference between an input signalreceived from the input node and an output signal which comes backthrough the feedback loop, and outputting a first output signal; acompensating amplifier unit coupled to the former stage amplifier unitfor amplifying the first output signal and outputting a second outputsignal; and an output stage amplifier unit coupled to the compensatingamplifier unit and the feedback loop for outputting a third outputsignal according to the second output signal.
 2. The amplifier circuitof claim 1, wherein the former stage amplifier unit is an integrator,and the former stage amplifier unit comprises: a first input nodecoupled to the feedback loop; a first output node for outputting thefirst output signal; a first impedance unit coupled to the input signaland the first input node for providing a first impedance value; and afirst capacitor unit coupled to the first input node and the firstoutput node for providing a first capacitor value.
 3. The amplifiercircuit of claim 2, wherein the compensating amplifier unit is anintegrator, and the compensating amplifier unit comprises: a secondinput node; a second output node for outputting the second outputsignal; a second impedance unit coupled to the first output node and thesecond input node for providing a second impedance value; and a secondcapacitor unit coupled to the second input node and the second outputnode for providing a second capacitor value.
 4. The amplifier circuit ofclaim 3, wherein the compensating amplifier unit further comprises athird impedance unit coupled to the second capacitor unit for providinga third impedance value.
 5. The amplifier circuit of claim 4, whereinthe product of the third impedance value and the second capacitor valueis less than
 1. 6. The amplifier circuit of claim 1, wherein thecompensating amplifier unit is an integrator, and the compensatingamplifier unit comprises: an input node; an output node for outputtingthe second output signal; a first impedance unit coupled to the inputnode of the integrator for providing a first impedance value; and acapacitor unit coupled to the input node of the integrator and theoutput node of the integrator for providing a capacitor value.
 7. Theamplifier circuit of claim 6, wherein the compensating amplifier unitfurther comprises a second impedance unit coupled to the capacitor unitfor providing a second impedance value.
 8. The amplifier circuit ofclaim 7, wherein the product of the second impedance value and thecapacitor value is less than
 1. 9. The amplifier circuit of claim 7,wherein the output stage amplifier unit is a switching amplifier. 10.The amplifier circuit of claim 1, wherein the feedback loop comprises animpedance unit for providing a feedback impedance value.
 11. Anamplifier circuit, comprising: a feedback loop; a first integratorcoupled to an input node of the amplifier circuit and the feedback loopfor amplifying the difference between an input signal received from theinput node and an output signal which comes back through the feedbackloop, and outputting a first output signal; a second integrator coupledto the first integrator for amplifying the first output signal andoutputting a second output signal; and a switch amplifier coupled to thesecond integrator and the feedback loop for outputting a third outputsignal according to the second output signal.